1. Field of the Invention
The present invention relates to a method and anapparatus for nuclear magnetic resonance imaging (referred hereafter as MRI) which utilizes nuclear magnetic resonance (referred hereafter as NMR) phenomena to obtain tomographic images of a body to be examined and, more particularly, to generating of gradient magnetic fields in such a method and an apparatus to be used with an ultra high speed imaging method such as an echo planar method.
2. Description of the Background Art
In order to apply an ultra high speed imaging method, the best known of which is an echo planar method, it is necessary to generate time-varying gradient magnetic fields of approximately 1 KHz frequency with strength several times stronger than those required by an ordinary imaging method such as a spin echo method, for the purpose of reading encoded information from NMR signals.
To achieve this, an amplifier of linear amplification type (referred hereafter as a linear type amplifier) ordinarily employed by an MRI apparatus to be used with an ordinary imaging method needs to possess an output capacity significantly more than ten times larger than usual, which is highly impractical.
On the other hand, there is a proposition to utilize an amplifier of resonant amplification type (referred hereafter as a resonant type amplifier) for this purpose, as it can generate strong enough magnetic fields while keeping its output capacitance reasonably small. This can be achieved by connecting a capacitor of an appropriate charge capacity, either in series or in parallel, between a gradient field coils which act as loading coils and the amplifier itself and thereby producing sinusoidal resonances.
However, with such a resonant type amplifier as an amplifier for the gradient fields for reading, it is not possible to obtain images at arbitrary cross sectional planes (called also slices) other than an axial plane A, a coronal plane C, and a sagittal plane S shown in FIG. 1, for the following reason.
In general, a cross sectional plane for MRI is specified in terms of a tilt angle .THETA. and a slew angle .PHI. defined as shown in FIG. 2(A) and FIG. 2(B), respectively.
Now, in order to obtain images at arbitrary cross sectional planes, three components Gx(t), Gy(t), and Gz(t) of the time-varying gradient magnetic field in three orthogonal directions X, Y, and Z, respectively, are required to be generated in sequences (referred hereafter as pulse sequences) according to the expression: ##EQU1## where Gx.sub.0 (t), Gy.sub.0 (t), and Gz.sub.0 (t) are the values of the components Gx(t), Gy(t), and Gz(t), respectively, with the tilt angle .THETA.=0 and the slew angle .PHI.=0. Example of pulse sequences Gx.sub.0 (t), Gy.sub.0 (t), and Gz.sub.0 (t) for the tilt angle .THETA.=0 and the slew angle .PHI.=0 are shown in FIG. 3 in which Gx.sub.0 (t) represents a pulse sequence for encoding, Gy.sub.0 (t) represents a pulse sequence for reading, and Gz.sub.0 (t) represents a pulse sequence for slicing. Here, the role of the pulse sequences Gx.sub.0 (t), and Gy.sub.0 (t) are exchangeable.
It is to be noted that although the above expression and the pulse sequences are expressed in terms of strength of the magnetic field, they can be considered as indicating amounts of current to be produced by the gradient field amplifier as they are proportional to each other.
Thus, when the ultra speed imaging method is to be carried out by a conventional MRI apparatus equiped with a gradient field amplifier having three channels, one of the three channels must be reserved for resonant type of amplification to produce a pulse sequence for reading, and this makes the generating of the aforementioned pulse sequences in three orthogonal directions impossible so that images are not obtainable at arbitrary cross sectional planes.
Moreover, when switching the imaging method between the ultra high speed imaging method and the ordinary imaging method in such a conventional MRI apparatus, a time-consuming cable connection change associated with the replacement of the resonant type amplifier by that of the linear type is necessary.